The usefulness of the phenomenon of magnetostrictivity in linear distance or position measuring devices is recognized by the prior art; for example, see Redding, U.S. Pat. No. 4,305,283; McCrea et al, U.S. Pat. No. 4,158,964; Krisst, U.S. Pat. No. 4,071,818; Edwards, U.S. Pat. No. 4,028,619; and Tellerman, U.S. Pat. No. 3,898,555. Common to several of these devices are a magnetostrictive wire that runs in a straight line path through the measurement field, means for inducing a torsional strain at a given position along the wire, and a magnet that is displaceable along the wire, either by connection to a movable body such as a machine tool or by reason of association with a float device. Generally speaking, the position of the magnet represents the position of the monitored quantity and is determined as a function of the time required for a torsional strain to propagate from one end of the wire through the area of influence of the magnet or from the position of the magnet to a sensing apparatus located at one end of the wire.
It is known in the art to provide increased position detection sensitivity and cancellation of differential circuit delays using a reflection technique. The magnetostrictive wire has a reflection termination at its foot end and a reflection damping termination at its head end. The position detection is made from the time between the receipt of the direct torsional strain and the receipt of the torsional strain reflected from the foot end. Position resolution is doubled because of the increased propagation path with this technique. In addition, circuit delays in detection of the direct and reflected responses are the same and are effectively canceled out.
There is a problem with this prior art technique. Prior art magnetostrictive position detectors employ a magnet with a single radially disposed pole. The single radially disposed pole magnet of the prior art produces identical torsional strain pulses traveling in both directions. The torsional strain generally has the form of a damped sine wave. The reflection of the torsional strain from the reflection termination inverts its phase. The electrical detector coupled to the transducer thus must detect different portions of the direct and reflected waveforms. It is difficult to provide reliable detection at the same location of the two damped sine waves due to their differing shape. It is possible to provide an electrical phase inversion of the reflected response before detection. However, this negates the advantage of the reflection technique in canceling out delays in the electrical system. This is because the two waveforms must be processed differently and thus could encounter differing delays under differing conditions resulting in potentially unreliable position detection. Therefore there is a need in the art to provide a manner of obtaining the full benefits of the reflection technique.